Last Updated: 30.03.2022

If you are old enough, you must remember the days when we used water to cool our engines.

Nowadays, modern car engines need a slightly more complex liquid that has superior cooling features compared to water. It’s called a coolant, which can be green, turquoise, yellow, or orange, depending on the type. This unique substance keeps engines running at optimum temperatures without them blowing up.

If your cooling system is full of coolant, it will resist overheating much longer. So, it’s vitally important to check the coolant regularly. But do you know how much coolant should be in the reservoir?

Keep on reading to find out!

What Is a Coolant Reservoir?

coolant reservoir inside the engine room

The main purpose of a coolant reservoir is to help prevent the loss of coolant due to overheating as the engine is running. In other words, as the coolant boils, it’s collected in the bottle.

When the engine cools, the coolant is recovered by the radiator. A coolant reservoir keeps outside air from being drawn into the engine block. Outside air may carry contaminants that can lead to corrosion in the cooling system. If contaminated air was to get into the engine block, it may cause airlocks as well as rusting.

Reservoirs come in a wide variety of structures, ranging from full plastic tanks or sleek cylindrical tubes that can be mounted right alongside your radiator. 

What Is an Engine Coolant?

pouring coolant into reservoir

Your vehicle’s engine is simply an internal combustion unit where power is produced through the expansion of high-pressure and high-temperature gases.

A lot of heat is produced during this process due to the friction of mechanical parts and combustion gas. For optimal operation, an engine coolant or antifreeze is used to help remove the excess temperatures. An engine antifreeze does a great job of dissipating heat.

The reason we use coolant and not water is that water freezes when it gets too cold and it boils when it gets too hot. Mixing coolant with water helps lower the freezing point and raise the boiling point of the liquid.

How Engine Coolant Works

how an engine coolant works

The coolant is sent through the passages in your vehicle’s engine heads and block, picking up the excess generated heat.

Then the heated fluid is sent to the radiator through a rubber hose. The radiator is located in front of your vehicle. When the heated fluid is flowing through the radiator, it is cooled down by the air that’s getting into your engine through the grill.

The cooled fluid then flows back to your vehicle’s engine to absorb excess heat. The fluid’s movement is controlled by the water pump. 

Types of Engine Coolant

coolant with different colors

Not many car owners know the importance of coolant types available on today’s market. Coolants are available in more than one color, which can be rather puzzling for people new to this product.

We will tell you why coolant liquids come in different colors like yellow, orange, and green. But there is no universal color guide to coolants or antifreeze. So, you have to be familiar with the most common coolants available today.

Inorganic Acid Technology (IAT)

This is one of the oldest types of coolants, which is not usually recommended for brand new vehicles. It is typically green and contains phosphate and silica as corrosion additives.

The corrosion inhibitors help protect metal parts of your vehicle like the radiator. This inorganic coolant has a life of about 24,000 miles or an equivalent of two years. This coolant is normally used in older vehicles that were manufactured before the mid-1990s.

Organic Acid Technology (OAT)

This engine coolant doesn’t feature phosphates or silicates. But it features corrosion inhibitors that enable it to last for extended periods. It uses special additives to prevent corrosion and rusting.

Usually orange in appearance, this coolant is recommended for some newer engines that don’t necessarily need metal protection. With this type, you’ve got an organic rust inhibitor like 2-EHA, sebacate, or carboxylate. You should expect it to last up to 150,000 miles or an equivalent of 5 years before flushing it out of the system.

Hybrid Organic Acid Technology (HOAT)

This technology is a mix between organic acid and inorganic acid coolants. It is mainly recommended for newer vehicles out there.

It is basically an organic acid antifreeze with a small amount of silicate that improves metal protection and stops corrosion. This technology also contains additives for preventing and stopping rusting. The color of HOAT coolant usually varies from one manufacturer to another. But the most popular colors out there are red or violet.

Type A or B

Engine coolants are also classified into two large types: Type A and Type B.

Type A

Coolants in this type include inhibitors, which are for preventing rusting and scaling. They also contain anti-boil and anti-freeze (glycol), which helps raise the boiling temperature or lower the freezing temperature.

Type B

Type B coolants, on the other hand, only contain inhibitors for preventing rusting and scaling. 

So, How Much Coolant Should Be in the Reservoir?

The reservoir is usually found near the radiator. Unlike the pressure cap on the radiator, you can pop open the reservoir lid easily and safely.

It makes it much easier to add coolant and water as opposed to pouring the fluids into the radiator. You also don’t risk damaging the radiator of your vehicle.

Most reservoirs have minimum and maximum lines. But some coolant tanks may only have the fill line or minimum line. The level of the coolant should be at the minimum line or fill line (the recommended level when filling the coolant) when your vehicle is cool. When your engine is hot, the level of the coolant in the reservoir will be at the maximum line.

So, if you have been wondering how much coolant should be in the reservoir, just look for the minimum line or mark on the reservoir. 

Ideally, the tank should be filled around 30% full, which is the minimum line when the engine is cold.

How to Check Coolant Level

The most important step when checking the level of coolant in your vehicle is to allow your engine to cool down before doing anything. We strongly discourage you from opening the cap of the reservoir or radiator when the engine is hot. This could lead to serious injuries from burns.

Several identifiers can help you pinpoint your coolant system caps. Look for labels like coolant reservoir and verbiage that tells you what type of antifreeze your system requires. But the most important label or sign is the hot temperature warning that is usually displayed on most containers. The reservoir is usually a translucent tank near the radiator.

Once you locate the reservoir, you can visually inspect the level on the side of the tank. The level should be between the minimum and maximum lines. You might want to know – does engine coolant evaporate?

If the level is below the minimum line, you will need to top it up with more fluid. Always check your owner’s manual for the recommended type of coolant. 

One sign that the coolant level is low is if the car temperature gauge goes up and down while driving, although this symptom could be caused by other issues. 

Mistakes When Adding Coolant

These are the common mistakes you might make when checking and adding coolant.

Working With a Hot Engine

One of the biggest mistakes you can make when checking or adding coolant is working with a hot engine. You can suffer severe burn injuries when you attempt to open the cap of the overflow tank or radiator when your engine is hot. We recommend letting the engine cool down before doing anything.

Over Filling the Fluid

You might mistakenly fill the new coolant above the minimum line. If you do that the coolant may overflow when the engine gets too hot. This is so because the coolant usually expands when the engine is running.

Incorrect Engine Coolant

The use of incorrect engine coolant is increasingly a concern and one of the causes of engine glitches. In the past, generic coolant could be used to fill any cooling system because manufacturers’ engine coolant was mostly the same.

Nowadays, several different coolant chemistries are not compatible with each other or with green coolant.

Using incorrect coolant can cause components to deteriorate and then fail. The failures are normally associated with galvanic or cavitations corrosion.

Mixing Different Engine Coolants

It isn’t a very good idea to mix engine coolants of different colors. Several things can happen when you mix incompatible coolants.

First, you are likely to end up with a brownish mixture that doesn’t look right. 

Second, you are going to have particles moving inside your coolant system that could clog passageways, including the radiator heater core and other passages in the block. 

Third, the coolant will degrade, affecting its performance as far as raising or lowering the temperatures go.

Fourth, the additives responsible for preventing corrosion are likely to be contaminated, exposing your system to corrosion. So, this can be a very costly mistake, which you should avoid by all means.

What Causes Problems in the Coolant System?

Will your engine ever give you a sign that it’s developing a cooling system issue? Yes, and here are the most common ones:

Overflow Coolant 

This is usually a problem that occurs when you fill excess coolant into the reservoir (above the minimum, low, or fill line).

But if the problem occurs even if you didn’t overfill the reservoir, then something else is wrong with your coolant system. It may be due to combustion gases leaking into your engine coolant jackets or an air leak on the suction side of the water pump. You should inspect the lower radiator hose for any damage.

Coolant Leak

A coolant leak is more than a nuisance. It represents lost engine cooling capacity. Even if you continually top-up the correct amount when the system is low, the engine still runs hotter in summer.

Before you look anywhere else for a coolant leak, first inspect the reservoir carefully for a leaking crack. If the overflow tank is cracked, check the plastic molding seams. You may be able to save the cracked part with a thick film of epoxy.

Bad Radiator Cap

A bad radiator pressure cap cannot do its job of allowing pressure to build up in the cooling system. The build-up pressure serves to increase the boiling point of the coolant.

When the cap leaks pressure, the boiling point of the coolant will fall. To find out, attach the radiator pressure cap to the radiator pressure tester. Then pump up the tester until the dial reads the specification stamped on the cap. If the dial starts falling off, then it’s an indication that the cap is damaged and you need a new one.

You also need to check the radiator filler neck to make sure it isn’t damaged. A damaged filler neck will prevent the cap from seating firmly although the cap may be in good shape.


Can I add coolant directly into the radiator?

New radiators in modern vehicles are sealed. So, instead of adding antifreeze directly to the radiator, you add it into the reservoir (the engine has to be cold).

Given the fact that the radiator is sealed, you may get a bubble of air in the top that keeps the storage tank from adding coolant. This usually results in less coolant than the engine requires. If you suspect this kind of a problem, then wait for the engine to cool, force the top of the radiator out, and take a look inside.

If the antifreeze is not covering the internal fins, then you have found the problem. You can then add coolant directly to the opening in the radiator.

How can I tell the coolant is contaminated?

When checking the general condition of the existing coolant in your vehicle, look for signs of scale build-up around the filler neck. When you observe sludge in the coolant. A cracked cylinder head or bad head gasket may have allowed coolant and oil to mix. You could also observe reddish deposits that appear as slime due to corrosion.

The Bottom Line

How much coolant should be in the reservoir? The level should be approximately 30 percent full. In other words, it should be on the minimum line or low mark on your reservoir. If the coolant level, however, seems to drop abnormally, we suggest you check for a coolant leak, leaking combustion gases, or a bad radiator pressure cap.